The Global Positioning System (GPS) is a global navigation satellite system maintained by the United States government that provides continuous position and time information. The GPS includes an Earth-orbiting satellite constellation that broadcasts microwave signals. As is well known in the art, compatible GPS receiving devices receive and process the data in the signals to determine the three-dimensional position (latitude, longitude, and elevation) of the receiver.
GPS receivers typically only operate reliably outdoors where there is a clear line of sight to at least three or four of the satellites. The receiver cannot update the position information using GPS alone when trees, buildings, and other obstructions block, scatter or significantly attenuate the microwave signals. This is a concern for soldiers and others who require accurate location information in environments where GPS signals may not be available, such as in forests, caves, or mountainous regions. Techniques for augmenting GPS exist; however, these techniques typically are designed to correct for errors in the GPS signal and require another system to transmit additional position information to the receiver.